Grating dispersion compensator and method of manufacture

ABSTRACT

A grating dispersion compensator (GDC), including: a substrate; a dielectric grating layer; a planar waveguide; and a passivation layer; is disclosed. The dielectric grating layer may be formed on the substrate and includes a variation in refractive index. This variation in refractive index defines a grating period. The grating period may vary along the longitudinal axis of the GDC according to a predetermined function. A selected center wavelength and dispersion curve may be created. The chirp of the grating period may be controlled by current, voltage, temperature, or pressure. The planar waveguide is formed on the dielectric grating layer and includes an input/output (I/O) surface normal to the longitudinal axis of the planar waveguide. The passivation layer is formed on the planar waveguide. Alternatively, a GDC may be formed with the dielectric grating layer on top of the planar waveguide rather than beneath it.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/167,915, filed Jun. 11, 2002, which claims priority of U.S.Provisional Application No. 60/314,951, filed Aug. 24, 2001, thecontents of which are incorporated herein by reference.

This invention relates to grating dispersion compensators. Moreparticularly this invention relates to dispersion compensation gratingsintegrally formed within planar waveguide apparatuses.

BACKGROUND OF THE INVENTION

Optical communication systems use pulses of light to transmit data. Thelight used to create these pulses necessarily contains a band ofwavelengths, rather than a single wavelength, even though the light isgenerally from laser sources. A broadening of the wavelength band occurswhen the light is pulsed, due to conditions imposed by the Fouriertransform spectrum of the pulse shape. Some devices, such aselectro-absorption modulators (EAM's), used to generate the pulsesbroaden the wavelength band of the light even further than the transformlimit. The desire for ever higher speed optical communications requiresever shorter pulses of light, with ever larger wavelength bands. Aproblem related to this growing wavelength band of optical pulses istemporal broadening due to chromatic dispersion.

Many components of optical communications system, such as the opticalfibers, EAM's, semiconductor optical amplifiers (SOA's), and variableoptical attenuators (VOA's), have chromatic dispersion. The existence ofchromatic dispersion in the system means that different wavelengths oflight travel at different speeds within the system. This phenomenon maylead to temporal pulse broadening. For example, in SMF28 fiber,operating at 1550 nm, longer wavelengths of light travel more slowlywithin the optical fiber than shorter wavelengths. The dispersionfactor, D, in this example is 17 ps/nm/km. Therefore, the shorterwavelengths in a pulse move ahead of the longer wavelengths, broadeningthe pulse. As the length of the fiber increases this broadeningaccumulates and chromatic dispersion ultimately limits the maximumdistance a data stream can be transmitted before the pulses becomeindistinguishable due to broadening. For a transform-limited pulseoperating at 40 GB/S, the distance before the signal deterioratessignificantly due to chromatic dispersion is approximately 4 km.

It is, therefore, desirable to compensate for this chromatic dispersioneffect by introducing dispersion compensators, which have an oppositeeffect. A number of methods for introducing this dispersion compensationhave been used, such as dispersion compensating fibers (DCF's) or fiberBragg gratings. Generally, these methods are not easily tunable and,therefore, can only be used for compensating fixed dispersion values. Itis impractical to design a specific DCF or fiber Bragg grating for everypossible chromatic dispersion due to various lengths of optical fiber orother optical components in a communications signal. Also, in wavelengthdivision multiplex systems, owing to differences in the slope ofdispersion compensated fibers and standard fibers such as SMF28 fibers,each channel would need to be fine tuned individually.

In a reconfigurable optical communications network the chromaticdispersion may be varied when the path length changes due to cabledamage, overload, or other rerouting. This also may be a problem forfixed dispersion compensation devices such as DCF's or fiber Bragggratings.

U.S. Pat. No. 6,363,187 B1 to Fells et al. describes an opticalwaveguide provided with a linearly chirped Bragg reflective gratingwhich can be used to provide linear dispersion compensation. The amountof dispersion compensation provided by this device can be adjusted bychanging the magnitude of the axial strain imposed on the grating. Fellset al. disclose that, adjusting the linear dispersion of the grating inthis manner requires the presence of a quadratic chirp term, eitherwithin the grating itself or within the strain placed upon the grating.This quadratic chirp term leads to additional difficulties and,therefore, should be compensated, at least in part, by causing theoptical signal to be reflected by a second Bragg reflective grating witha quadratic component of chirp having an opposite sign to that of theoriginal Bragg reflective grating.

Fells et al. also note that similar effects may be achieved by adjustingthe effective refractive index of the waveguide grating structure bychanging the temperature of the waveguide, possibly creating atemperature gradient along the longitudinal access of the waveguide inthe process. In this temperature tunable dispersion compensating devicedisclosed by Fells et al., the temperature in both the original chirpedBragg reflective grating and the secondary Bragg reflective grating usedfor quadratic chirp term compensation should be varied simultaneously.

There is therefore a useful role for an adjustable amplitude lineardispersion compensation device. Such a device could be one designed foroperation on its own to achieve substantially complete dispersioncompensation. Alternatively, it could be one designed for operation inassociation with a fixed amplitude dispersion compensation device, suchas a length of DCF, that provides a level of compensation which isinadequately matched on its own. The adjustable device may be operatedwith some form of feedback control loop to provide active compensationthat can respond to dynamic changes of dispersion within the system, andin suitable circumstances to step changes resulting from re-routingoccasioned, for instance, by a partial failure of the system such as atransmission fiber break.

SUMMARY OF THE INVENTION

One embodiment of the present innovation is an exemplary gratingdispersion compensator (GDC), which includes a substrate, a dielectricgrating layer, a planar waveguide, and a passivation layer. Thedielectric grating layer is formed on the top surface of the substrateand includes a variation in index of refraction. This variation in indexof refraction defines a grating period. The grating period may varyalong the longitudinal axis of the GDC according to a predeterminedfunction. The variable (chirped) grating period causes differentwavelengths of light to have different transit times within the GDC. Aselected center wavelength and dispersion curve may be created. Theplanar waveguide is formed next on the dielectric grating layer andincludes an input/output (I/O) surface. The longitudinal axis is normalto the I/O surface of the planar waveguide. The passivation layer isformed on the planar waveguide. An alternative exemplary GDC may beformed in which the dielectric grating layer is formed on top of theplanar waveguide rather than beneath it.

In an alternative embodiments of the present invention, an effectivegrating period chirp may be formed by means other than varying theactual grating period of the dielectric grating layer. These alternativeembodiments allow an effective variation in the grating period whetherthe actual grating period of the dielectric grating layer varies or not.

One such alternative embodiment is to curve the planar waveguide. Thegrating period along the, curved, longitudinal axis of the curved planarwaveguide may then be varied as desired due to the variation in anglebetween the axis of the grating and the longitudinal axis of the planarwaveguide.

Another such alternative embodiment uses a variation in the index ofrefraction along the longitudinal axis of the planar waveguide to createan effective grating period chirp. This exemplary planar waveguide mayinclude a quantum well structure. The thickness of the waveguide may bevaried along the longitudinal axis as desired using selective areagrowth. This thickness variation changes the index of refraction of thewaveguide due to the quantum well structure.

Other embodiments of the present innovation are exemplary dynamicallytunable GDC's, including electrically-controlled GDC's,thermally-controlled GDC's, and pressure-controlled GDC's. Theseexemplary dynamically tunable GDC's include a substrate, a dielectricgrating layer, a planar waveguide, and a passivation layer. Thedielectric grating layer includes a variation in index of refraction,defining a grating period. The grating period may be constant, or mayvary along the longitudinal axis of the GDC according to a predeterminedfunction.

An exemplary electrically-controlled GDC includes at least one substrateelectrode coupled to the substrate and at least one top electrodecoupled to the passivation layer. By controlling the voltages and/orcurrents between these electrodes, the center wavelength and/or theshape of the dispersion curve of the GDC may be tuned.

An exemplary thermally-controlled GDC includes at least one heatingelement coupled to the front portion of the passivation layer, near theI/O surface of the waveguide. By controlling the temperature of theheating element(s) to create temperature gradients in the GDC, thecenter wavelength and the shape of the dispersion curve of the GDC maybe tuned. Exemplary heating elements may be resistive heating elementsor a thermo-electric coolers. In one alternative embodiment a resistiveheating element is formed over the passivation layer such that thelinear resistivity, and therefore the heat generated, varies along thelongitudinal axis. A first electrical contact is coupled to the frontportion of the resistive heating element, near the I/O surface of theplanar waveguide, and a second electrical contact is coupled to the backportion of the resistive heating element, near the other end of theplanar waveguide.

An exemplary pressure-controlled GDC includes a plurality of electrodescoupled to the sides of the passivation layer and at least a portion ofthe passivation layer includes a piezoelectric material electricallycoupled to the first electrode and the second electrode. By controllingthe internal pressure of created by the piezoelectric material in theGDC, the center wavelength and the shape of the dispersion curve of theGDC may be tuned.

A further embodiment of the present invention is an exemplary apodizedGDC. Exemplary apodized GDC's are formed by applying an apodizationtechnique to a previously described exemplary GDC. Exemplary apodizationtechniques include: forming a spacing layer with a variable thicknessbetween the dielectric grating layer and the waveguide; and varying adimension, either thickness or width, of the dielectric grating layeralong the longitudinal axis of the waveguide.

Yet another alternative embodiment of the present invention is anexemplary dispersion compensated optical signal detection apparatus. Theexemplary dispersion compensated optical signal detection apparatusincludes a circulator, an exemplary GDC of the present invention, and anoptical detector. The exemplary GDC and the optical detector areoptically coupled to the circulator.

A further alternative embodiment of the present invention is anexemplary dispersion compensated optical signal amplification apparatus.This dispersion compensated optical signal amplification apparatusincludes a first optical fiber, a circulator, an exemplary GDC of thepresent invention, a semiconductor optical amplifier (SOA), and a secondoptical fiber optically coupled to the SOA. The first optical fiber,exemplary GDC, and SOA are optically coupled to the circulator. Thesecond optical fiber is optically coupled to the SOA.

In an alternative exemplary embodiment of the dispersion compensatedoptical signal amplification apparatus, the SOA is between thecirculator and the exemplary GDC and the second optical fiber isoptically coupled to the circulator instead of the SOA.

Still another alternative embodiment of the present invention is anexemplary monolithic optical chip. An exemplary monolithic optical chipincludes a substrate, a waveguide layer, and a passivation layer. In theback portion of the exemplary monolithic optical chip, the substrateincludes a second layer, which forms an optical grating section, and thewaveguide layer includes a GDC waveguide section. These sections form aGDC. In the front portion of the exemplary monolithic optical chip asecond optical device is formed within the monolithic structure. Thissecond optical device may be an active optical device, or a passiveoptical device. For an exemplary embodiment including an active secondoptical device, the waveguide layer includes an active layer section ofplanar waveguide and an I/O surface at the front end of the monolithicoptical chip. A first electrode is coupled to the substrate and a secondelectrode coupled to the passivation layer over the active devicesection of the waveguide layer. For an exemplary embodiment includingthe passive second optical device, the waveguide layer includes an inputsurface, an output surface, and a circulator section within the frontportion of the monolithic optical chip.

Additional alternative embodiments of the present invention, includeexemplary methods of manufacturing exemplary GDC's. A first exemplarymethod of manufacture includes several steps. A substrate is provided. Agrating layer base is formed on the substrate, then defined and etchedto form a grating pattern. A grating top layer is formed over the etchedgrating base layer such that the grating top layer and the grating baselayer form a dielectric grating layer having an index of refraction thatchanges to define at least one grating period. A waveguide layer isformed over the dielectric grating layer and defined and etched to forma waveguide. A passivation layer is formed over the waveguide. Awaveguide input/output surface is then formed on the waveguide, often bycleaving the GDC from a wafer.

A second exemplary method of manufacture includes many of the samesteps. A substrate is provided. A waveguide layer is formed over thesubstrate and defined and etched to form a waveguide. A spacing layer isformed on the waveguide and the substrate. The spacing layer includes agrating layer base, which is defined and etched to form a gratingpattern. A grating top layer is formed over the etched grating baselayer such that the grating top layer and the grating base layer form adielectric grating layer having an index of refraction that changes todefine at least one grating period. A passivation layer is formed overthe grating top layer. A waveguide input/output surface is then formedon the waveguide, often by cleaving the GDC from a wafer.

BRIEF DESCRIPTION OF THE FIGURES

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1A is a top plan drawing of an exemplary GDC with a chirpedgrating.

FIG. 1B is a side plan drawing of an exemplary GDC with a chirpedgrating.

FIG. 2 is a graph illustrating compression of an exemplary optical pulsebroadened by chromatic dispersion.

FIG. 3 is a front plan drawing of an exemplary GDC.

FIG. 4 is a flowchart illustrating exemplary manufacture method of anexemplary GDC.

FIGS. 5A, 6A, 7A, and 8A are top plan drawings of an exemplary GDCduring manufacture according to the flowchart of FIG. 4.

FIGS. 5B, 6B, 7B, and 8B are side plan drawings of an exemplary GDCduring manufacture according to the flowchart of FIG. 4.

FIGS. 5C, 6C, 7C, and 8C are front plan drawings of an exemplary GDCduring manufacture according to the flowchart of FIG. 4.

FIGS. 9A and 9B are side plan cut-away drawings of alternative exemplaryGDC's fabricated using selective area growth (SAG) techniques forapodization.

FIG. 10 is a top plan drawing of an alternative exemplary GDC usinggrating width variation for apodization.

FIG. 11 is a top plan drawing of an alternative exemplary GDC includinga curve waveguide.

FIG. 12A is a side plan cut-away drawing of an exemplarycurrent-controlled dynamically tunable GDC.

FIG. 12B is a side plan cut-away drawing of an exemplaryvoltage-controlled dynamically tunable GDC.

FIGS. 13A, and 13B are side plan cut-away drawings of exemplarytemperature-controlled dynamically tunable GDC's.

FIG. 13C is a top plan drawing of an exemplary temperature-controlleddynamically tunable GDC.

FIG. 13D is a side plan drawing of the exemplary temperature-controlleddynamically tunable GDC of FIG. 13C.

FIG. 14A is a top plan drawing of an exemplary pressure-controlleddynamically tunable GDC.

FIG. 14B is a side plan drawing of an exemplary pressure-controlleddynamically tunable GDC.

FIG. 14C is a front plan drawing of an exemplary pressure-controlleddynamically tunable GDC.

FIG. 15 is a block diagram illustrating an exemplary opticalcommunications detection system including a GDC.

FIG. 16 is a block diagram illustrating an exemplary extended rangeoptical communications system including a GDC.

FIG. 17 is a side plan cut-away drawing of an exemplary GDC with avariable thickness quantum well waveguide.

FIG. 18 is a flowchart illustrating alternative exemplary manufacturemethod of an exemplary GDC.

FIGS. 19A, 20A, and 21A are top plan drawings of an exemplary GDC duringmanufacture according to the flowchart of FIG. 18.

FIGS. 19B, 20B, and 21B are side plan drawings of an exemplary GDCduring manufacture according to the flowchart of FIG. 18.

FIGS. 19C, 20C, and 21C are front plan drawings of an exemplary GDCduring manufacture according to the flowchart of FIG. 18.

FIG. 22A is a side plan drawing of an exemplary monolithic optical chipincluding a GDC.

FIG. 22B is a top plan drawing of an exemplary monolithic optical chipincluding a GDC.

DETAILED DESCRIPTION OF THE INVENTION

One exemplary embodiment of the present invention, illustrated in FIGS.1A and 1B, is GDC 100 including chirped optical grating 104. FIG. 1A isa top plan drawing and FIG. 1B is a side plan drawing of GDC 100. Theexemplary device includes substrate 102, chirped optical grating 104,wave guide 106, and passivation layer 108. Waveguide 106 desirablyexhibits low optical loss and a relatively high index of refraction inthe desired wavelength band. Although the exemplary devices described inthe present application are formed from various III/V materials, such asInP, GaAs, AlGaAs, or InGaAsP, other possible materials choices arecontemplated. Possible alternative materials for waveguide 106 include:doped silica (similar to optical fibers); silicon; germanium; and otherdielectric materials, which have low optical loss characteristics forthe desired wavelength band and a relatively easily variable index ofrefraction.

Passivation layer 108 and chirped optical grating 104, and preferablysubstrate 102 as well, have a lower index of refraction than waveguide106 and act as cladding layers. The materials for these three layers maydesirably be selected from a related family of materials to the materialused to form waveguide layer 106. Such a material selection may minimizescattering at the boundaries between layers and may also improve thequality of crystalline materials by reducing lattice mismatches betweenlayers.

Additionally, waveguide 106 may contain a number of sub-layers, forminga quantum well structure within this layer. A quantum well waveguidestructure may be desirable to increase the tunability of the index ofrefraction within the waveguide. This structure may include a singlequantum well, multiple quantum wells, or separate confinement layers.Substrate layer 102 and passivation layer 108 may also contain aplurality of sub-layers. It is often desirable for passivation layer 108to extend around the waveguide on the sides as well as over the top ofwaveguide 106, as shown in FIG. 8C.

In addition to serving as a cladding layer, passivation layer 108 mayalso desirably function as the p-type material of a P-I-N structure. Aquantum well structure may be particularly desirable for exemplarytunable GDC's, such as those described below with respect to FIGS.12A-14C.

FIGS. 1A and 1B also illustrate how an incident optical signal 109having a number of different wavelengths λ₁, λ₂, λ₃, λ₄, andλ₅(represented by arrows 110, 112, 114, 116, and 118) is reflected byexemplary GDC 100. It is noted that the inclusion of five distinctwavelengths is merely for illustration purposes as is the spatialseparation of the arrows 110, 112, 114, 116, and 118. Chirped opticalgrating 106 includes a periodic structure formed of two materials withdiffering Indices of refraction. It is also noted that the periodicstructure of optical grating 106 is shown to include only gratingperiods for each exemplary wavelength. This extremely limited number ofgrating periods is merely for illustrative purposes. It is alsocontemplated that the variation of grating periods may be smooth ratherthan a stepped function, as shown. It is further noted thatrepresentation of the periodic structure of optical grating 106 shown inFIGS. 1A and 1B is somewhat abstracted. This periodic structure may beformed individual grating elements of a higher index of refractionformed with a material having a lower index of refraction. Alternativelythe periodic structure may be a variation in the thickness of twomaterials of optical grating 106. This variation may resemble a squarewave, a triangular wave, a sine wave, or other periodic oscillation.

As shown, it is generally desirable for chirp of optical grating 106 toprogress from larger spacings in the index of refraction variation,known as the grating period, near input/output (I/O) surface 107 ofwaveguide 106, to narrower spacings along the longitudinal axis ofdevice 100, i.e. the propagation axis of waveguide 106. This arrangementleads longer wavelengths to be reflected closer to I/O surface 107 andtherefore to have a short transit time within exemplary GDC 100. Thisgrating period chirp may be linear, or it may be determined by apredetermined function to more accurately match the chromatic dispersionto be compensated.

FIG. 2 illustrates the effect of the difference in transit times onexemplary chromatically dispersed optical signal 109. Curve 200illustrates an exemplary dispersion broadened pulse before enteringexemplary GDC 100 and curve 202 illustrated the recompressed pulseexiting the device.

Even though the majority of the light is confined within the waveguide,proper selection of indices of refraction in the device may desirablyallow enough of the optical field to extend into the chirped opticalgrating for Bragg reflection to occur at resonant wavelengths, similarto methods employed to create feedback in distributed feedback (DFB)lasers. Resonant wavelengths are those wavelengths where an integralnumber of wavelengths are substantially equal to twice the gratingperiod. The grating period is based on the effective optical distancebetween the alternating material portions of the grating element. Theperiod of chirped optical grating 104 is varied along the longitudinalaxis of the device. Therefore, different wavelengths are reflected atdifferent distances due to diffraction from chirped optical grating 104.Desirably, larger grating periods occur near the input of the device andthe first order Bragg reflection is significant. Such a chirped opticalgrating design may help to limit losses due to secondary reflectionsback into the device, as well as substantially limiting reflections tofirst-order Bragg reflections only. Apodization methods, described belowwith respect to FIGS. 9A, 9B, and 10 may be used as well.

Alternatively, it is contemplated that a negative chirp may be createdin the chirped optical grating, with shorter grating periods near theinput of the device. Such a negatively chirped GDC may be used to finetune over-compensated systems, or may be tailored to other applicationsfor which negative dispersion compensation may be desirable. Therelatively narrow bandwidth most pulsed laser systems desirably reducesthe effect of second-order, or higher, Bragg reflections.

It is not always practical to couple light emitted from optical fibersinto a planar waveguide as either a pure TE or TM mode. Therefore it isdesirable to account for both TE and TM modes during design.Inefficiencies may occur in exemplary GDC 100 due to possible dispersionbetween TE and TM modes in waveguide 106. It is, therefore, desirable tolimit this polarization modal dispersion. Designing waveguide 106 tohave an approximately square cross-section, as shown in FIG. 3, is onemethod, which may be used to limit dispersion between TE and TM modes inwaveguide 106.

FIG. 3 illustrates a front side view of exemplary GDC 100. Grating layer104 from FIGS. 1A and 1B is shown as two sub-elements, spacing layer 302formed of a first material and grating element 300 formed of a secondmaterial having a higher index of refraction than the material ofspacing layer 302. Spacing layer 302 is desirably formed of the samematerial as substrate 102 to avoid possible unwanted scattering due toan index of refraction difference between the spacing layer and thesubstrate. The difference in the indices of refraction of gratingelement 300 and spacing layer 302 desirably provide the small amount ofscattering which leads to Bragg reflections in exemplary GDC 100. Theamount of scattering may be controlled by a number of variables,including: the indices of refraction of grating element 300 and spacinglayer 302; the thicknesses of grating element 300 and spacing layer 302;and the width of grating element 300. The width of grating element 300may desirably be greater than the width of waveguide 106 in the mainreflecting portion of the device. The thicknesses and indices ofrefraction of the grating layer sub-elements may be optimized for thewavelengths desired to be compensated. The material family desired to beused in the device may also affect this optimization.

The passivation layer is also shown as two sub-elements, sidepassivation layers 304 and top passivation layer 306. It may bedesirable for these passivation layer sub-elements to have differentcompositions in exemplary tunable GDC's described below, particularlythose shown based on electrical tuning (FIGS. 12A and 12B) and pressuretuning (FIGS. 14A-C).

Exemplary GDC 100 may be designed for 1.55 μm optical signals, usingIII/V materials. Such a device may be formed on an InP substrate 102.Spacing layer 302, side passivation layers 304, and top passivationlayer 306 are also desirably formed of InP. Grating element 300 andwaveguide 106 are desirably formed of InGaAsP material. The exemplarybandgap of grating element 300 is desirably 1.1 μm, while waveguide 106desirably has a bandgap of 1.0 μm. Waveguide 106, in this exemplaryembodiment, is 0.9 μm×0.9 μm. Grating element 300 are 3.0 μm×0.1 μm.Spacing layer 302 has a thickness of a 0.5 μm between waveguide 106 andgrating element 300. This exemplary GDC desirably provides a periodicperturbation in the effective index of refraction along waveguide 106 ofapproximately 5×10⁻⁴.

FIG. 4 is a flowchart describing an exemplary technique for producing aGDC according to the present invention. FIGS. 5A-C, 6A-C, 7A-C, and 8A-Cillustrate various steps of this exemplary fabrication process. Althoughit is contemplated that other materials may be used, this exemplarymethod is described in terms of III/V semiconductor materials.

The process begins with a substrate, step 400. Substrate 102, shown inFIGS. 1A, 1B, and 3, may function as both a cladding layer to assist incontainment of the beam in the device and as the N layer of the P-I-N,possibly quantum well, diode structure or may function as only acladding layer. (Although this description assumes that the substrate isthe N side of the P-I-N structure, one skilled in the art willunderstand that the substrate could be the P side with the passivationlayer 108 formed of N-type material instead.) The substrate in thisexemplary device is preferably formed of a III/V semiconductor, such asInP, GaAs, InGaAsP, InGaAs, or InAlGaAs. The substrate may also beformed of multiple layers such as GaAs grown on silicon or alumina.

Next grating base layer 500 is grown, step 402. Metal organic chemicalvapor deposition (MOCVD) is the preferred method for deposition of thissub-layer, but other epitaxial deposition techniques may also beemployed, such as molecular beam epitaxy (MBE), chemical beam epitaxy(CBE), and liquid phase epitaxy (LPE). This layer is the basis of thegrating element and desirably is composed of InGaAsP with an index ofrefraction higher than that of the substrate.

Selective area growth (SAG) may be used to grow a layer of varyingthickness. Therefore, the thickness of grating base layer 500 can easilybe modified through the use of SAG to help provide apodizatlon of thegrating device, which is described below with reference to FIG. 9A. Thethickness variation in the grating base layer 500 may be desirablygentle enough to be adiabatic, thereby diminishing the possibility ofunwanted scattering losses within an exemplary GDC using this technique.

To perform SAG of grating base layer 500 a patterned growth-retardinglayer is formed on the top surface of the substrate before growinggrating base layer 500. Materials which retard growth of III/Vmaterials, such as SiN or SiO₂, make up the growth-retarding layer. Thegrowth-retarding layer may be formed and patterned using any standardtechniques known in the semiconductor industry. In the areas covered bythe growth retarding layer growth is substantially inhibited andmaterial which falls upon the growth retarding layer diffuses laterallyacross the surface. Near the growth-retarding regions the growth rate isenhanced owing to gas phase diffusion and surface diffusion of thereactants in the MOCVD, or other epitaxial, reactor away fromgrowth-retarding regions. After grating base layer 500 is grown thegrowth retard areas may be removed. Thickness variations of 5:1 orgreater are attainable in this manner.

The grating base layer is defined and etched to form the grating elementin step 404. A resist layer is applied on top of grating base layer 500and patterned, as shown in FIGS. 5A-C. The pattern for this resist layermay be written using techniques similar to those employed in theproduction of distributed feedback (DFB) lasers, such as e-beamlithography. Alternatively, resist layer pattern 502 may be formed usinga modified holographic technique. The holographic pattern is firstformed as a fringe pattern between interfering light beams. As theholographic pattern is swept across the device to expose the resist, thedevice may be moved in a controlled manner by a precise movement stage,such as a piezoelectric solenoids, to alter the apparent spacing betweenfringes relative to the surface of the resist.

The grating base layer is then etched, and the resist is removed. Thisetch procedure preferably uses a wet etch technique, but a dry etch orcombination may be employed. A wet etch may provide less abrupt edges.Grating base layer 500 may be etched to form grating element 600 with anundulating surface, such as shown in FIGS. 6A and 6B, or the layer maybe etched completely though to the base layer forming a plurality ofindividual grating sub-elements. Separating the grating element intoindividual sections in this manner may be particularly desirable forgrating base layers formed using SAG. Also it may be desirable to etch,or form, grating base layer 500 so that a narrower grating element,which does not extend over the full width of substrate 102, may beformed. Grating element 300 in FIG. 3 and tapered grating element 1000in FIG. 10 illustrate two possible exemplary narrower grating elements.

In an alternative embodiment, the forming, defining, and etching gratingbase layer 500, steps 402 and 404, may be performed by an SAG technique.Patterned growth-retarding regions may be formed to induce grating baselayer 500 to be formed with a desirably undulating surface so that noetching is needed. A grating base layer formed in this manner mayadditionally be designed for apodization at the same time.

FIGS. 1A and 1B illustrate an exemplary GDC with a chirped opticalgrating, as do FIGS. 8A-C. The various alternative exemplary tunableGDC's illustrated in FIGS. 11, 12A, 12B, 13A, 13B, 14A-C, and 17 may beformed with a chirped optical grating as well, or they may be formedwith a grating having a constant spacing. If a grating having a constantspacing is desired then the pattern defined in the resist layer has aconstant spacing. The only step of the exemplary method of FIG. 4effected by this difference is step 404 (shown in FIG. 4).

Next, grating top layer 602 (shown in phantom), which includes thespacing layer, is formed over grating element 600 (shown in FIG. 6), andany exposed sections of the top surface of substrate 102, step 406. Itmay be preferable for the grating top layer to be formed of the samematerial as substrate 102. The grating top layer desirably has a lowerindex of refraction than the grating element 600. This layer may bepreferably formed using an epitaxial technique. The surface may then beplanarized to provide an improved surface upon which to form thewaveguide layer.

It may also be desirable to use SAG techniques, described above withrespect to step 402, on the grating top layer to produce a variation inthe thickness of the spacing layer to assist in apodization of theexemplary device. Exemplary GDC's including variable thickness spacinglayer 905 are shown in FIGS. 9A and 9B. If SAG is used to form avariable thickness spacing layer, then the top grating layer may not beplanarized.

Once grating layer 104 is completed at step 406, waveguide layer 700 isformed over grating layer 104, preferably by an epitaxial growthtechnique, step 408. FIGS. 7A-C Illustrate the partially completedexemplary GDC following step 408. (If the thickness of grating layer 104is varied using SAG, then waveguide layer 700 will be bowed.) Exemplarywaveguide layer 700 is preferably formed of InGaAsP having an index ofrefraction higher than the refractive index of the grating top layer.Waveguide layer 700 may consist of a single layer of bulk material, adual layer having one n-doped portion and one p-doped portion, as shownin FIG. 12B, or a plurality of layers forming a quantum well structure.A single layer of bulk material may be the waveguide layer type mosteasily grown. The dual layer may provide advantages for electricaltunability as described below with respect to FIG. 12B.

The growth of a quantum well structure in the waveguide layer mayprovide a waveguide that is more sensitive to electrical, temperature,and/or pressure variations. It is contemplated that this increasedsensitivity may be exploited to increase the range, and possibly speed,of dynamic tunability which may be achieved in exemplary tunable GDC's,such as those shown in FIGS. 11, 12A, 12B, 13A, 13B, and 14A-C.Additionally, by using SAG, it is possible to grow a single multi-layerquantum well structure of varying thickness. Possible advantages of sucha quantum well structure are described below with respect to FIG. 17.

Waveguide layer 700 is next defined and etched to form waveguide 106,step 410. Wet or dry etch techniques may be used. Alternatively, it maybe desirable to first dry etch a structure slightly larger than thedesired waveguide dimensions and then use a selective InGaAsP undercutetch to achieve the desired size for waveguide 106. This two partetching technique may provide a highly controllable method to formwaveguide 106. As noted above, the cross-sectional shape and size ofwaveguide 106 may be important to minimize polarization modaldispersion. FIG. 11 illustrates curved waveguide 1102, which may be usedto control the grating chirp of an exemplary GDC.

Next passivation layer 108 is formed over waveguide 106, step 412. Thisstep of the fabrication process is illustrated in FIGS. 8A-C.Preferably, passivation layer 108 is formed using the same method andmaterial as the grating top layer. Passivation layer 108 desirably has arefractive index lower than waveguide 106, preferably similar to that ofsubstrate 102 and the grating top layer, to act as a cladding layer andensure light confinement within waveguide 106. Additionally, passivationlayer 108 may be formed of a p-doped material, preferably InP or GaAs.Also, the passlvation layer may be formed in multiple sub-layers. Ifeither grating layer 104 or waveguide 106 are formed using SAG it may bedesirable to planarize passivation layer 108, as illustrated in FIGS.9A, 9B, and 17.

Following step 412, FIG. 4 includes three alternative exemplaryprocedures which may be used to complete alternative exemplaryembodiments of GDC's according to the present invention. The simplest ofthese alternative procedures is proceed directly to final step 414 andcomplete the exemplary GDC by forming waveguide I/O surface 107. Thissurface may be formed by any methods commonly used in semiconductorlaser manufacture, such as cleaving the wafer in which the exemplary GDChas been formed to separate the device. Alternatively, ananti-reflection (AR) coating (not shown) may be deposited on I/O surface107 to improve optical coupling of the device to other optical devices.This deposition may be accomplished by a number of methods known tothose skilled in the art, such as e-beam evaporation, vapor phasedeposition or sputtering. Static exemplary GDC's, such as those shown inFIGS. 1A and 1B, 8A-C, and 11 may be formed in this manner.

Two alternative procedures, also shown in FIG. 4, may be used to createexemplary dynamically tunable GDC's. These exemplary dynamically tunableGDC's may allow for dynamic tuning of both the center wavelength and thedispersion characteristics of the device across the wavelength band thatis to be compensated. FIGS. 12A, 12B, and 14A-C illustrate exemplarydynamically tunable GDC's formed by the second alternative procedure.Following formation of passivation layer 108 electrical contacts on thepassivation layer and/or substrate 102 at step 416.

The electrical contacts formed on the electrically tunable exemplaryGDC's of FIGS. 12A and 12B may include: substrate electrical contact1200, coupled to substrate 102; resistive contact layer 1202, coupled topassivation layer 108; and first electrical contact 1204 and secondelectrical contact 1206 which may be coupled to resistive contact layer1202 or passivation layer 108. It is noted that additional contactssimilar to first contact 1204 and second contact 1206 may be connectedalong top of the device or substrate contact 1200 may be connected tovoltages other than ground, as shown, and may be formed in severalsections.

As shown in FIG. 14, electrical contacts 1400 maybe formed only onpassivation layer 108 of the pressure tunable exemplary GDC.

These contacts may be formed using standard semiconductor fabricationtechniques. The dynamically tunable exemplary dispersion compensatinggrating device is then completed by forming the waveguide I/O surface,step 414 as described above.

The final alternative exemplary procedure included in FIG. 4 may be usedto create a temperature tunable exemplary dispersion compensatinggrating device, such as those shown in FIGS. 13A and 13B. Followingformation of the passivation layer, step 412, a heating element isformed on the passivation layer, step 418. The heating element ispreferably resistive heater composed of a metal, such as Al, Au, Cu, Ag,Ti, W, or an alloy, formed using a deposition technique, though it iscontemplated that semiconductor resistive heaters or thermo-electriccoolers may be used instead. FIG. 13A shows a single heating element1300 located near the front of the exemplary temperature tunable GDC(i.e. the same side as I/O surface 107). Additional heating elements maybe added to provide greater control over the temperature profile of theexemplary temperature tunable GDC. FIG. 13B shows graduated heatingelement 1304 with electrical contacts 1306 at either end of theexemplary GDC. FIGS. 13C and 13D illustrate alternative graduatedheating element 1312. It is noted that, although graduated heatingelements 1304 and 1312 are shown with linear gradients, other gradientsare possible. Lithographic techniques to form complex width variationsin graduated heating element 1312 may be used.

Next, heatsink 1302 is thermally coupled to substrate 102, step 420. Theheatsink may also be coupled the back side of the exemplary temperaturetunable GDC as shown in FIG. 13B. Preferably a pre-fabricated heatsinkis attached to the temperature tunable GDC with thermally conductiveepoxy, a layer of indium, or another standard method, but it is alsocontemplated that a heatsink may be formed by etching a metal layerdeposited directly on the bottom surface of substrate 102.

This alternative exemplary procedure also ends with the formation ofwaveguide I/O surface 107. It is noted that step 420 may be performedduring packaging of the exemplary temperature tunable GDC, after step414.

FIG. 18 is a flowchart describing an alternative exemplary technique forproducing an exemplary GDC in which the optical grating is formed abovethe waveguide, rather than below it. FIGS. 19A-C, 20A-C, and 21A-CIllustrate various steps of this exemplary fabrication process. Many ofthe steps are the same as the steps of the exemplary method of FIG. 4.It is contemplated that the various alternative embodiments contemplatedfor exemplary GDC's produced based on the exemplary manhood of FIG. 4may also be practiced for exemplary GDC's produce using the exemplarymethod of FIG. 18.

The process begins with a substrate, step 1800. Substrate 102, shown inFIGS. 19A-C, may function as both a cladding layer to assist incontainment of the beam in the device and as the N layer of the P-I-Ndiode structure or may function as only a cladding layer. The substratein this exemplary device is preferably formed of a III/V semiconductor,such as InP, GaAs, InGaAsP, InGaAs, or InAlGaAs. The substrate may alsobe formed of multiple layers such as GaAs grown on silicon or alumina.

First, the waveguide layer is formed over substrate 102, preferably byan epitaxial growth technique, step 1802. The waveguide layer ispreferably formed of InGaAsP having an index of refraction higher thanthe refractive index of the substrate. The waveguide layer may consistof a single layer of bulk material, a dual layer of with one n-dopedportion and one p-doped portion or a plurality of layers forming asingle or multiple quantum well structure.

The waveguide layer is next defined and etched to form waveguide 106,step 1804. Wet or dry etch techniques may be used. Alternatively, asdescribed above, it may be desirable to first dry etch a structureslightly larger than the desired wavegulde dimensions and then use aselective InGaAsP undercut etch to achieve the desired size forwaveguide 106. This two part etching technique may provide a highlycontrollable method to form waveguide 106.

Next, spacing layer 1900 which includes the grating base layer is grown,step 1806. MOCVD is the preferred method for deposition of thissub-layer, but other epitaxial deposition techniques may also beemployed, such as MBE, CBE, and LPE. This layer is the base of thegrating element and desirably is composed of InP with an index ofrefraction similar to that of the substrate. The top surface of spacinglayer 1900 is planarized, step 1808, to prepare for etching the gratingbase layer.

The grating base layer is defined and etched to form grating element2000 in step 1810. A resist layer is applied on top of the grating baselayer and patterned, as shown in FIGS. 19A-C. The pattern for thisresist layer may be written using techniques such as e-beam lithographyand holographic techniques, as described above.

The grating base layer is then etched, and the resist is removed forminggrating element 2000, shown in FIG. 20. This etch procedure preferablyuses a wet etch technique, but a dry etch or combination may beemployed. A wet etch may provide less abrupt edges. FIGS. 19A-Cillustrate the partially completed exemplary GDC following step 1810. Athin portion of spacing layer 1900 may desirably remain betweenwaveguide 106 and grating element 2000.

Next, the grating top layer (not shown in FIGS. 20A-C), is formed overgrating element 2000, and any exposed sections of the top surface ofwaveguide 106, step 1812. It may be preferable for the grating top layerto be formed of InGaAsP with an index of refraction between the indicesof refraction of the waveguide and the spacing layer. This layer may bepreferably formed using an epitaxial technique. The surface may then beplanarized.

Next passivation layer 108 is formed over optical grating 104, step1814. Preferably, passivation layer 108 is formed using the same methodand material as spacing layer 1900. Passivation layer 108 desirably hasa refractive index lower than that of waveguide 106, preferably similarto that of substrate 102 and spacing layer 1900, to act as a claddinglayer and ensure light confinement within waveguide 106.

Exemplary GDC 2100, shown in FIGS. 21A-C, may be completed by formingwaveguide I/0 surface 107, step 1816. This surface may be formed by anymethods commonly used in semiconductor laser manufacture, such ascleaving the wafer in which the exemplary GDC has been formed toseparate the device. Alternatively, an anti-reflection coating may bedeposited on I/O surface 107 improve optical coupling of the device toother optical devices. This deposition may be accomplished by a numberof methods known to those skilled in the art, such as e-beamevaporation, vapor phase deposition or sputtering.

It is desirable that the interaction between the grating element and theoptical signal travelling within the waveguide of an exemplary GDCdecrease near the ends of the exemplary GDC. This apodization may beused to provide improved dispersion characteristics in the exemplaryGDC. An exemplary apodization profile that may desirably be used is asuper-Gaussian profile. Using this profile the interaction between thegrating element and the optical signal is approximately zero at thefront and back ends of the GDC waveguide. This interaction increases asa Gaussian toward the middle of the waveguide, but rather than peakingin a single position at the center, the super-Gaussian shape has anextended flat peak throughout the central portion of the waveguide

FIGS. 9A, 9B, and 10 illustrate several exemplary apodization techniquesthat may be utilized in an exemplary GDC. FIGS. 9A and 9B are side planviews of an exemplary GDC and FIG. 10 is a top plan view of an exemplaryGDC.

The apodization method illustrated in FIG. 9A involves varying theseparation between waveguide 904 and grating element 600. The intensityof the optical signal tail within spacing layer 905 decreases rapidlywith distance from the waveguide edge. Therefore, there is desirablyvery little interaction with the grating element near the ends of thisexemplary GDC, i.e. outside of lines 900 and 902. Thickness variationsof 3:1 within spacing layer 905 may provide a reasonable level ofapodization, but variations of 5:1 may be preferable. It may bedesirable for the central portion (between lines 900 and 902) ofwaveguide 904 to be relatively straight, but it may also be desirablefor the maximum curvature of waveguide 904 to remain low so as tominimize any effects on waveguide performance. This may affect the totallength of the exemplary GDC. The preferred method to accomplish thethickness variation of spacing layer 905 is through SAG during growth ofthis layer. Etching techniques may also be used.

FIG. 9B illustrates an alternative apodization technique for varying thethickness of grating element 906. Thinner grating element regions nearthe exemplary GDC ends provide less interaction with the optical signal.Although the exemplary GDC shown in FIG. 9B utilizes variation of thethickness of spacing layer 905 as well as variation of the thickness ofgrating element 906 for apodization, it is contemplated that sufficientapodization may be accomplished by varying the thickness of gratingelement 906 alone. As shown in FIG. 9B, it may be preferable to havegrating element 906 narrowest at both ends of the exemplary device andthickest in the middle of the longitudinal axis, but it may also bedesirable for the central portion of grating element 906 to be flat oreven somewhat narrow. It may also be desirable for grating element 906to be narrow only toward the end of the exemplary GDC, which includesI/O surface 107.

FIG. 10 illustrates a third exemplary apodization technique. Thisapodization technique reduces interaction between the optical signal andgrating element 1000 by narrowing the grating element near the ends ofthe exemplary GDC. Although not necessary, it may be preferable tonarrow grating element 1000 to zero width at the ends of the exemplaryGDC, as shown in FIG. 10.

The three exemplary apodization techniques described above may be usedindividually or may be combined. All three techniques may be use withgrating elements that have a constant longitudinal spacing or withgrating element that have a variable longitudinal spacing as illustratedin FIGS. 1A, 1B, and 8A-C. Also, these techniques may be used witheither the exemplary static GDC's or the exemplary dynamically tunableGDC's described above. It is noted that the chromatic dispersion ofexemplary apodized GDC's may be designed such that the desiredwavelength band is reflected between line 900 and line 902, shown inFIG. 9, to maintain a relatively constant effective reflectivitythroughout the band. This may also reduce the amount of apodizationdesired at the back end of the exemplary apodized GDC due to thedesirably low amount of light transmitted past line 902.

FIG. 11 illustrates an alternative exemplary method of providing thegrating chirp in a GDC. The exemplary embodiment in FIG. 11 uses curvedwaveguide 1102 and angled optical grating 1100 to vary the effectiveperiod of the grating element along the, curved, longitudinal axis ofthe waveguide. The specific shape of curved waveguide 1102, and hencethe effective chirp of this exemplary GDC, may be controlled with greatprecision. Maximum curvature of the waveguide may be limited bypolarization modal dispersion between the TE and TM mode of thewaveguide. Angled optical grating 1100 may also be formed with a chirpedperiod to allow greater chromatic dispersion and possibly additionalcontrol of the effective chirp, or it may be formed to have a constantperiod, as shown in FIG. 11. An exemplary GDC design incorporating acurved waveguide and an angled grating element with a constant periodmay be more easily fabricated than an exemplary GDC with a straightwaveguide and a chirped optical grating.

FIG. 17 illustrates exemplary GDC 1700 in which quantum well waveguide1702 includes a quantum well structure of varying thickness. It is alsonoted that the bandgap, and hence the cutoff wavelength, of a quantumwell structure depends, among other factors, on the thickness of themultiple layers within the quantum well structure. Near the cutoffwavelength the index of refraction varies significantly as a function ofwavelength. According to the Kramers-Kronig relationship, if the cutoffwavelength is varied, then there is also a variation in the wavelengthdependence of the index of refraction. Therefore, using SAG, it ispossible to design a GDC in which the index of refraction for a range ofwavelengths varies in a predictable manner along the longitudinal axisof quantum well waveguide 1702 by varying the thickness of the quantumwell layers forming waveguide 1702.

The period of optical grating element 104 experienced by optical signal109 depends on the effective index of refraction, n_(eff), of the GDC.The value of n_(eff) is perturbed slightly by the indices of refractionof optical grating element 104 and passivation layer 108, but is largelydetermined by the index of refraction of quantum well waveguide 1702.This allows wavelength dependent chirp of the dispersion curve ofexemplary GDC 1700 to be controlled by varying the thickness of quantumwell waveguide 1702. It is contemplated that the period optical gratingelement 104 may be constant, or may be chirped. It is also contemplatedthat quantum well waveguide 1702 may be curved in the same way as curvedwaveguide 1102 in FIG. 11.

The quantum wells and barriers of quantum well waveguide 1702 arepreferably In_(x)Ga_((1−x)) _(As) _(y)P_((1−y)) materials as well asIn_(x)Al_(y)Ga_((1−x))As_((1−y)) and In_(x)Ga_((1−x))As materials.Specific selections of x and y are dependent on the desired bandgap andstrain, if any, desired. These sub-layers may also be formed by otherpermutations of alloys formed from these elements.

Static GDC's are passive optical devices and, therefore, have no powerconsumption and require very minimal maintenance once they areinstalled. Dynamically tunable GDC's, however, may offer a number ofadvantages over static GDC's. Dynamically tunable GDC's may be designedto provide dynamic tuning of the center wavelength, or they may bedesigned to provide dynamic tuning of the dispersion characteristics ofthe device across the wavelength band desired to be compensated, orboth.

This means that a dynamically tunable GDC may provide superiorperformance by tracking shifts in the optical wavelength due to age orenvironmental conditions. A common use for dispersion compensationelements is to compensate for the broadening of signal pulse due tochromatic dispersion from transmission over long-haul optical fibers, orfrom non-linear optical media in various optical devices. The shape, andmagnitude, of the chromatic dispersion curve is dependent on a number ofparameters, such as the length of the optical fiber, specific to eachapplication. It is impractical to produce a static GDC for eachindividual use. One solution may be to produce standard sized staticGDC's, generally with linear dispersion curves, and selected the closestone. With a dynamically tunable GDC, the dispersion characteristics mayalso be tuned during installation, or operation, to optimize performancein the specific application the dynamically tunable GDC is used in.Also, changes in the system configuration, such as adding a kilometer ofoptical fiber or a new semiconductor optical amplifier (SOA), may becompensated by changing the tuning of the dynamically tunable GDC.

FIGS. 12A, 12B, 13A, 13B, and 14A-C illustrate several exemplarydynamically tunable GDC's. These devices are preferably operated bydynamically varying n_(eff) of the waveguide, and thus the effectiveoptical period of the optical grating. A small change in the actualperiod of the optical grating due to thermal expansion, piezoelectric,and/or electrostriction effects may also occur. These devices may bedesigned with optical grating elements having a constant period and thedispersion chirp desirably added by varying n_(eff) along thelongitudinal axis of the waveguide. The center wavelength may also befine-tuned. Alternatively, the actual optical grating period may bechirped, or a curved waveguide as shown in FIG. 11 may be used, and thetuning applied for fine-tuning of the center wavelength and/ordispersion curve. Additionally, it is contemplated that an exemplarydynamically tunable GDC may be tuned using combinations of theseexemplary methods, such using both electrical and thermal effects, tocontrol n_(eff) of the waveguide. An exemplary GDC using a combinationof effects may have advantages due to differences in how each methodeffects the wavelength dependence of the index of refraction. Near anabsorption peak the chromatic dispersion of n_(eff) may be significant.

Quantum well structures, including variable thickness quantum wellstructures such as those of quantum well waveguide 1702 of FIG. 17, maybe desirable for the waveguide of an exemplary dynamically tunable GDCdue to the increased sensitivity of these structures.

FIG. 12A illustrates an exemplary current-controlled GDC. Changes incarrier density may be used to alter the bandgap and, hence, the indexof refraction of semiconductor materials. Therefore, the n_(eff) ofwaveguide 106 may be varied by passing a current between resistivecontact layer 1202, which preferably has a significantly lowerelectrical resistance than passivation layer 108, and substrateelectrode 1200. The amount of current flow at various positions alongthe longitudinal axis of the exemplary GDC may be controlled by thevoltages placed on first electrode 1204 and second electrode 1206. Ashift in the center wavelength may be accomplished by varying thevoltage in both first electrode 1204 and second electrode 1206 equally.A longitudinal variation in current density, leading to changes in theshape of the dispersion curve, may be accomplished by varying thevoltage of first electrode 1204 and second electrode 1206 relative toone another. It is contemplated that the current may flow in eitherdirection.

The longitudinal variation in current density may also be manipulated byvarying the thickness, and in turn the voltage of distribution, ofresistive contact layer 1202. It is noted that resistive contact layer1202 may also be omitted and the longitudinal spread of the currentdensity may occur substantially in passivation layer 108. It is alsonoted that the choice to couple two electrodes to the top side of theexemplary current tunable GDC is merely illustrative. A greater numberof electrodes may allow more sensitive control of the dispersion curve.Alternatively, it may be desirable to include only one electrode and tohave the longitudinal current density distribution determined by thelinear resistivity of resistive contact layer 1202 or the other layersof the exemplary current tunable GDC. Although, waveguide 106 may be asingle layer bulk semiconductor material, it is contemplated that it maybe desirable for waveguide 106 to be a dual layer material formed with ap-doped layer and an n-doped layer operating like a forward-biased diodejunction, or a quantum well structure.

FIG. 12B illustrates an exemplary voltage-controlled GDC. This exemplarytype of dynamically tunable GDC is very similar to thecurrent-controlled GDC described above with respect to FIG. 12A. Themajor difference is the use of a dual layer waveguide structure composedof n-doped layer 1210 and p-doped layer 1212 as a reverse biased diodejunction. (It may also be desirable that passivation layer 108 andsubstrate are p-doped and n-doped, respectively.) A reverse biasedquantum well structure may also be employed. The intended effect of thereverse-biased junction is to limit current flow and, thereby, reducepower consumption. The variation in n_(eff) is accomplished bycontrolling the carrier depletion region of doped layer 1210 and 1212.One possible depletion region profile is outlined by dashed lines 1214.The Franz-Keldysh effect may also play a role in varying n_(eff) in thisexemplary voltage-controlled GDC.

FIGS. 13A and 13B illustrate alternative exemplary embodiments fortemperature controlled GDC's. The relatively simple embodiment in FIG.13A uses heating element 1300 and heat sink 1302 to create a temperaturegradient within waveguide 106. This temperature gradient may be used tocreate a longitudinal variation in n_(eff). The temperature dependenceof bulk InGaAsP is approximately 1.9×10⁻⁴K⁻¹. Therefore, a temperaturegradient of 100° C. between I/O surface 107 and the back end of theexemplary GDC may change n_(eff) on the order of 1%. A 1% change inn_(eff) may cause significant dispersion, even if optical gratingelement 104 has a constant period. Greater thermal sensitivity may bepossible in temperature-controlled quantum well waveguide structures.

A longitudinally uniform change in temperature along the waveguide mayhave the effect of shifting the center wavelength of the GDC.

Heating element 1300 may be a thin film of deposited metal, which mayonly raise the temperature of the GDC above the ambient temperaturethrough resistive heating, or it may be one or more thermo-electriccoolers capable of raising and/or lowering the temperature as desired.In this relatively simple embodiment, it may be desirable to change tooverall temperature of the GDC significantly from the ambienttemperature to lower effects due to changes in the ambient temperature.

Finer control of the temperature gradient profile may be attained by theaddition of further heating or cooling elements. Temperature sensors1310, as shown in FIG. 13B, may be added to the embodiment of FIG. 13Ato increase the precision and stability of temperature control byutilizing feedback control techniques. Also, heatsink 1302 may be movedand coupled to the back surface of the exemplary GDC in FIG. 13A (orextended to cover the back surface in addition to the bottom surface asis shown for alternative heatsink 1308 in FIG. 13B). Such an alternativeheatsink configuration may reduce the heat necessary to create thetemperature gradient along the longitudinal axis.

The alternative exemplary temperature controlled GDC embodiment shown inFIG. 13B includes graduated heating element 1304 with electricalcontacts 1306, temperature sensors 1310, and alternative heatsink 1308.Graduated heating element 1304 is preferably formed of a deposited metalfilm. By varying the thickness of this metal film along the longitudinalaxis of the GDC, the linear resistivity of graduated heating element1304 may also be varied along the longitudinal axis. The linear currentdensity remains constant. Therefore, according to Ohm's law, the voltagedrop varies with the resistance. Because the resistive heating of thiselement is equal to the current times the voltage drop, the amount ofheat conducted into the passivation layer varies with the profile of thegraduated heating element. The graduated heating element profile may bedesigned to provide a linear or a non-linear dispersion curve, asdesired. Graduated heating element 1304 may even be flat to provide aconstant temperature profile. Also, it is noted that alternativeheatsink 1308 may be replaced with heatsink 1302 shown in FIG. 13A.

FIGS. 13C and 13D illustrate an alternative embodiment of a thermallycontrolled GDC, similar to the exemplary embodiment of FIG. 13B. Theexemplary embodiment of FIGS. 13C and 13D includes alternative graduatedheating element 1312. Alternative graduated heating element 1312 ispreferably formed of a deposited metal film. By varying the width ofthis metal film along the longitudinal axis of the GDC, the linearresistivity of, and heat produced by, alternative graduated heatingelement 1312 may be varied along the longitudinal axis. The graduatedheating element width may be varied to provide a linear or a non-lineardispersion curve, as desired. Graduated heating element 1312 may also berectangular to provide a constant temperature profile.

Although FIG. 13B shows a single graduated heating element 1304extending over the entire length of the top surface of passivation layer108, it is contemplated that graduated heating element 1304 may extendover only a portion of the top surface of passivation layer 108. It isalso contemplated that alternative graduated heating element 1312 mayextend over less than the full length of passivation layer 108. If thegraduated heating element covers less than the whole top surface ofpassivation layer 108, then more than one graduated heating element maybe used and the multiple graduated heating elements may have differentlinear resistivity profiles as well. Alternatively, both the thicknessand width of a graduated heating element may be varied to create adesired temperature profile. The number and position of temperaturesensors 1310 may be changed as well. Utilizing these alternativeembodiments, greater tunability, or more complex dispersioncharacteristics, may be attained.

FIGS. 14A-C illustrate an exemplary pressure tunable GDC. In thisexemplary embodiment n_(eff) of the GDC is tuned using pressure onwaveguide 106 passivation layer 108 is formed of a piezoelectricmaterial, such as quartz, rochelle salt, potassium dihydrogen phosphate,and ammonium dihydrogen phosphate. Pressure may be generated byselective piezoelectric expansion, or contraction, of passivation layer108 using electrodes 1400. This exemplary piezoelectric expansion, orcontraction, is illustrated by arrows 1402. Voltages may be placed onthe various electrodes 1400 to create a specific dispersion curve. It isnoted that the choice of 16 electrodes is merely illustrative.

Alternatively, waveguide 106 may be formed of a piezoelectric material,or only a sub-layer of piezoelectric may be formed within thepassivation layer to craft the pressure due to expansion, orcontraction.

It is also contemplated that electrodes 1400 may be configured toprovide substantially vertical, rather than substantially horizontal,pressure on waveguide 106. This alternative method of pressure tuningn_(eff) may be particularly effective for waveguides with quantum wellstructures. A small change in the thickness of the quantum well layersmay significantly change the bandgap of such a structure. Shifting thebandgap effects both the absorption spectrum of the structure, but basedon the Kramers-Kronig relationship the wavelength dependent index ofrefraction as well.

One exemplary use of a GDC is shown in FIG. 15. The signal pulses of anoptical signal transmitted within fiber optics system 1500 may besignificantly broadened due to chromatic dispersion during transmission.This chromatic dispersion may result from transmission in optical fibersor may result from interaction with optical components such as EAM's,SOA's, or VOA's that may be included in fiber optics system 1500. Thebroadened signal pulses may become difficult for detector 1510 todistinguish. Therefore, it may be desirable to recompress the signalpulses by reversing the chromatic dispersion of the signal using GDC 100before detection.

The broadened optical signal is coupled from fiber optics system 1500into circulator 1504 via optical coupler 1502. Circulator 1504 routesthe broadened optical signal through I/O coupler 1506 and into GDC 100.The optical signal is substantially recompressed in GDC 100 andreflected back into I/O coupler 1506. Circulator 1504 routes therecompressed optical signal into detector 1510 through optical coupler1508. I/O coupler 1506 and optical couplers 1502 and 1508 may includeoptical fibers, planar waveguides, and/or lens systems.

Circulator 1504, I/O coupler 1506, and GDC 100 may be formed together asa monolithic dispersion compensation optical chip. Additionally, opticalcoupler 1508 and detector 1510 may be added to the optical chip to forma monolithic, dispersion compensated, optical signal detector.

The range over which an optical signal can be transmitted in such asystem is limited by losses as well as pulse broadening within long-hauloptical fibers. One solution is to detect the signal before it becomesundetectable and then retransmit the signal. This slows the overalltransmission speed of the system and may introduce errors. FIG. 16illustrates an exemplary extended range fiber optic communicationsystem, which employs SOA 1602, to extend the range betweenretransmissions.

The broadened optical signal is coupled from long-haul optical fiber1600 into circulator 1504 via optical coupler 1502. As in the detectorsystem of FIG. 15, circulator 1504 routes the broadened optical signalthrough I/O coupler 1506 and into GDC 100. The optical signal issubstantially recompressed in GDC 100 and reflected back into I/0coupler 1506. Circulator 1504 routes the recompressed optical signalinto SOA 1602 through optical coupler 1508. The recompressed andamplified signal is the coupled, through optical coupler 1604, intoanother long-haul optical fiber 1606 to continue its transit.

As an alternative embodiment, it may be desirable to place SOA 1602before circulator 1504. This embodiment may allow chromatic dispersionwhich may be introduced by SOA 1602 to be compensated by GDC 100, aswell as chromatic dispersion from long-haul optical fiber 1600. If thepulse broadening is too great, SOA 1600 not be able to operateefficiently in this configuration.

Another alternative embodiment of the exemplary extended range fiberoptic communication system of FIG. 16 involves placing SOA 1602 betweencirculator 1504 and GDC 100. This configuration allows the signal topass through SOA 1602 twice, providing additional amplification for theoptical signal. GDC 100 and SOA 1602 may be formed in this configurationas a single monolithic dispersion compensating amplifier chip, as shownin FIGS. 22A and 22B.

FIGS. 22A and 22B illustrates an exemplary monolithic optical chip inwhich transmissive planar waveguide optical component 2201 is integratedwith GDC 2200 in a single optical chip. GDC 2200, which includes aportion of substrate 102, optical grating 104, GDC waveguide 2206, and aportion of passivation layer 108, may be formed using any of thepreviously described embodiments.

Transmissive planar waveguide optical component 2201 may be a passiveoptical element, such as a circulator, or may be an active opticalelement, such as a variable optical attenuation (VOA), anelectro-absorption modulator (EAM), or an SOA, as shown in FIGS. 22A and22B. If transmissive planar waveguide optical component 2201 is acirculator, then monolithic optical chip 2200 may be formed with both aninput surface and an output surface in the front portion of the chip.Alternatively, both a circulator and an active optical element may beincluded in an exemplary monolithic optical chip. In this alternativeexemplary embodiment, the planar waveguide of the active optical elementmay be formed between the circulator and the GDC, or between thecirculator and either the input surface or the output surface of theexemplary monolithic optical chip.

The active optical element 2201 of exemplary monolithic optical chipshown in FIGS. 22A and 228 includes a portion of substrate 102, activewaveguide layer 2202, a portion of passivation layer 108 and topelectrode 2204. Active waveguide layer 2202 may a bulk material, or maybe a quantum well structure. It may be desirable for active waveguidelayer 2202 and GDC waveguide 2206 to be formed as a single layer toreduce possible boundary scattering. Therefore, if active waveguidelayer 2202 is formed as a quantum well structure, it may be preferablefor GDC waveguide 2206 to be formed to include the same quantum wellsub-layers as well.

Performance of quantum well structures in both GDC's and active opticalelements, such as SOA's and VOA's, may be greatly affected by thethickness of the sub-layers of the quantum well structure. SAG may beused to produce a quantum well structure in active waveguide layer 2202that has a different thickness than in GDC waveguide 2206. By producingdifferent thicknesses for active waveguide layer 2202 and GDC waveguide2206, each portion of the waveguide may be optimized. The waveguideportion between active waveguide layer 2202 and GDC waveguide 2206 maybe gradually changed to allow adiabatic expansion and contraction of theoptical signal modes, reducing any possible loss.

Although the embodiments of the invention described above have beenmostly in terms of GDC's formed in III/V materials and with the opticalgrating formed beneath the waveguide, it is contemplated that similarconcepts may be practiced with other dielectric materials, or may bepracticed with the optical grating formed on top of the waveguide. Also,it will be understood to one skilled in the art that a number of othermodifications exist which do not deviate from the scope of the presentinvention as defined by the appended claims.

1. A method of manufacturing a grating dispersion compensator,comprising the steps of: a) providing a substrate having a substrateindex of refraction; b) forming a grating layer base on the substratehaving a grating base index of refraction greater than the substrateindex of refraction on a portion of the top surface of the substrate; c)defining and etching a grating pattern in the grating layer base; d)forming a grating top layer having a grating top index of refractionless than the grating base index of refraction over the etched gratinglayer base such that the grating top layer and the grating layer baseform a dielectric grating layer having an index of refraction thatchanges to define at least one grating period; e) forming a waveguidelayer having a waveguide index of refraction greater than the substrateindex of refraction and greater than the grating top index of refractionover the dielectric grating layer; f) defining and etching a waveguidehaving a longitudinal axis in the waveguide layer; g) forming apassivation layer having a passivation index of refraction substantiallyequal to the substrate index of refraction over the waveguide; and h)forming a waveguide to form grating dispersion compensator input/output(I/O) surface on the waveguide.
 2. The method of claim 1, wherein step(b) includes the steps of b1) forming at least one patterned growthretarding layer on the substrate; b2) forming the grating layer base byselective area growth, the grating layer layer base having a changingthickness along the longitudinal axis, the thickness defined by a firstpredetermined function of position along the longitudinal axis; and b3)removing the at least one patterned growth retarding layer.
 3. Themethod of claim 1, wherein the at least one grating period formed insteps (b) and (c) is a first predetermined function of position alongthe longitudinal axis.
 4. The method of claim 1, wherein the gratingpattern formed in step (b) includes a variable width, and the variablewidth is a first predetermined function of position along thelongitudinal axis.
 5. The method of claim 1, wherein step (d) furtherincludes the steps of: d1) forming a first grating top sub-layer formingthe variation in index of refraction d2) forming at least one patternedgrowth retarding layer on the first grating top sub-layer; d3) forming aspacing sub-layer by selective area over the first grating topsub-layer, the spacing sub-layer including a variable thickness, and thevariable thickness being a first predetermined function of positionalong the longitudinal axis; and d4) removing the at least one patternedgrowth retarding layer.
 6. The method of claim 1, wherein step (e)further includes the step of: e1) forming the waveguide layer as aplurality of sub-layers forming a quantum well structure.
 7. The methodof claim 1, wherein step (e) further includes the steps of: e1) formingat least one patterned growth retarding layer on the grating top layer;e2) forming the waveguide layer by selective area over the first gratingtop sub-layer, the waveguide layer including a plurality of sub-layersforming a quantum well structure having a variable thickness, thevariable thickness defined by a first predetermined function of positionalong the longitudinal axis; and e3) removing the at least one patternedgrowth retarding layer.
 8. The method of claim 1, further comprising thestep of: i) forming an anti-reflective coating on the waveguide I/Osurface.
 9. The method of claim 1, further comprising the step of: i)forming at least one substrate electrode on the substrate; and j)forming at least one top electrode on the passivation layer.
 10. Themethod of claim 1, further comprising the step of: i) forming at leastone substrate electrode on the substrate; j) forming a resistive contactlayer on the passivation layer; and k) forming at least one topelectrode on the resistive contact layer.
 11. The method of claim 1,further comprising the step of: i) forming a plurality of top electrodeson the passivation layer; wherein the passivation layer formed in step(g) includes a sub-layer of piezoelectric material.
 12. The method ofclaim 1, further comprising the step of: i) forming at least one heatingelement on the passivation layer.
 13. The method of claim 12, furthercomprising the step of: j) coupling a heating sink to the substrate. 14.The method of claim 12, further comprising the step of: j) coupling atleast one temperature sensor to the substrate.
 15. The method of claim12, further comprising the step of: j) coupling at least one substrateelectrode to the substrate; wherein the heating element formed in step(i) is electrically coupled to the passivation layer.
 16. A method ofmanufacturing a grating dispersion compensator, comprising the steps of:a) providing a substrate; b) forming a waveguide layer over thesubstrate; c) defining and etching a waveguide in the waveguide layer;d) forming a spacing layer on the waveguide and the substrate, thespacing layer including a grating layer base; e) defining and etchingthe grating layer base; f) forming the grating top layer over the etchgrating-base-layer such that the grating top layer and the gratingbase-layer form a dielectric grating layer having an index of refractionthat changes to define at least one grating period; g) forming apassivation layer over the dielectric grating layer; and h) forming awaveguide to form grating dispersion compensator input/output surface onthe waveguide.
 17. A method of manufacturing a grating dispersioncompensator, comprising the steps of: a) providing a substrate having asubstrate index of refraction; b) forming a plurality of patternedgrowth retarding layers on the substrate; c) forming a grating layerbase on the substrate having a grating base index of refraction greaterthan the substrate index of refraction on a portion of the top surfaceof the substrate by selective area growth, the grating layer base layerhaving a changing thickness along the longitudinal axis, the thicknessdefining a grating pattern in the grating layer base; d) removing the atleast one patterned growth retarding layer; e) forming a grating toplayer having a grating top index of refraction less than the gratingbase index of refraction over the etched grating layer base such thatthe grating top layer and the grating layer base form a dielectricgrating layer having an index of refraction that changes to define atleast one grating period; f) forming a waveguide layer having awaveguide index of refraction greater than the substrate index ofrefraction and greater than the grating top index of refraction over thedielectric grating layer; g) defining and etching a waveguide having alongitudinal axis in the waveguide layer; h) forming a passivation layerhaving a passivation index of refraction substantially equal to thesubstrate index of refraction over the waveguide; and i) forming awaveguide to form grating dispersion compensator input/output (I/O)surface on the waveguide.